The present invention is directed to a composition capable of imparting wet strength to cellulosic sheet products and to the resultant product.
"Cellulosic sheet materials" and "paper products" are terms used herein to include all sheet and sheet like products made from or containing cellulosic fibrous materials. Such products include paper, cardboard, corrogated board and the like.
Cellulosic fibers are hydrophilic and hence are readily wetted and swollen by water. The surfaces of these swollen fibers, when compacted into a web as in paper sheet formation, come together in close contact. As water is removed by drying, interfiber bonding forces develop between the adjacent surfaces. This bonding provides strength and coherence to the finished sheet. Conversely, when such a sheet is rewetted, the fibers swell, the interfiber bonding is weakened, and the paper loses most of its strength. Precise testing of various papers made from different pulps and with various manufacturing conditions but without artificial bonding shows a range of tensile strength retention from about 1 to about 8 percent on complete saturation with water. Until comparatively recently, most paper was used for writing, printing, and illustrating. For such uses the property of wet strength was relatively unimportant. As paper increased in abundance and was applied for a wide variety of uses, it became apparent that some of these uses would be benefited if the paper possessed greater strength when wet.
A series of discoveries in the later 1930s and early 1940s opened the door to mass production of wet-strength paper. It was shown that the use of certain thermosetting resins, particularly the aminoplasts, urea-formaldehyde (UF) and melamine-formaldehyde (MF), dramatically improved the wet strength of paper. These resins were applied to the fiber in the water-soluble, monomeric, or intermediate stage of polymerization. Subsequently, the adsorbed resin was cured to the insoluble condition and wet strength became evident. In the early days of this development, the resins were applied to the already formed sheet as an impregnating solution. Soon, however, resins were developed that were substantive to cellulosic fibers. Such resins could be added to paper stock prior to sheet formation, be adsorbed by the fiber, and eventually cured to produce the wet-strength property.
A wet-strength paper product shows extraordinary resistance to rupture or disintegration when saturated with water. Wet strength is distinguished from water repellancy or resistance to wetting. To the extent that a paper remains unwetted, even in contact with water, strength will be retained. All papers can be wetted under proper conditions, and to measure wet strength, the wetting liquid must be water.
The three major types of resins in current use are urea-formaldehyde resin, melamine-formaldehyde resin and polyamide-polyamine-epichlorohydrin (PPE) resins. The former two require acidic conditions which are normally associated with high degrees of corrosion in the paper machine system and to paper products of inferior nature. The PPE resins carry large amounts of amide groups particularly in the polymer backbone which, although aiding in imparting water solubility to the resin, does not effectively aid in wet strength. Overall, these polymers must be used in high dosages to provide any desired degree of wet strength.
There is a need to have resin which is capable of being incorporated into a paper product and effectively imparting a high degree of wet strength to the product even when used as low dosages than some of the previous known and used compounds.